At the end of one of the Northern Hemisphere’s wildest winters in memory, we thought it would be a good time to ask a climatologist what’s up with such extreme weather. Even Australia’s normally calm summer has been anything but: First there was drought, then typhoons and then floods of biblical proportions.
Granted, such extreme weather has been exacerbated both by recent El Niños (warmer-than-normal sea surface temperatures) and La Niñas (colder-than-normal sea surface temperatures) in the equatorial Pacific. But it does give pause to wonder: Is this global warming on steroids, or are we merely seeing natural cyclical weather variations?
For answers, we turned to Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colo. The climatologist, a leading member on the Intergovernmental Panel on Climate Change, shared the Nobel Peace Prize in 2007 when it was awarded to the IPCC.
He recently addressed this strange weather in a question-and-answer session with Miller-McCune.com.
Kevin Trenberth: Natural variability plays a major role along with human-induced climate change. There have been strong El Niño to La Niña transitions and natural modes of variability, such as the North Atlantic Oscillation, that have played major roles. During the latter part of El Niño and the following five months, heat comes out of the ocean and contributes to warming and is working in the same direction as global warming from humans. The result has been record-breaking sea surface temperatures and thus moisture in the atmosphere.
M-M: Is this related to a more southerly trajectory of the Northern Hemisphere’s jetstream?
KT: That’s correct. It is related to the negative phase of the NAO, which allows cold polar outbreaks to occur more readily. The NAO largely varies through natural processes on weekly, monthly or even longer timescales.
M-M: Does this negative phase of the NAO allow colder air to come to more southerly latitudes?
KT: In the negative phase, the westerlies [anti-trade winds] across the North Atlantic are weaker than normal and the cold air over the Arctic can flow more readily into lower latitudes and vice versa. In particular, there are often cold outbreaks over Europe with high temperatures over northeastern Canada and Greenland. That was the case in early January.
M-M: What’s causing the NAO’s negative phase?
KT: That’s largely a natural variability, relating to interactions between individual storms and the overall westerlies in the atmosphere. There is a region of stronger westerlies in the positive in the NAO’s positive phase and weaker westerlies in the NAO’s negative phase.
M-M: Does climate change or global warming exacerbate these NAOs?
KT: That’s a $64,000 question. In general, under global climate change, it’s suspected that the NAO becomes a little more positive. But in the last two years, it’s become quite negative, and so the speculation is about whether the negative phase is in any way related to the decrease in Arctic sea ice.
M-M: Are these current weather events anomalous?
KT: One measure of how anomalous things are is whether you’re breaking records. There have been more record-breaking events than expected. In the U.S., for instance, the number of high temperature events has more than doubled [in the first decade of the 21st century]. What’s happened is that the ratio of record-breaking hot events to cold events is about 2 to 1.
M-M: At what point would you be concerned about the freak weather signaling something truly deleterious about our climate?
KT: I already am. Natural variability is going on all the time and the biggest effects are from the El Niño. Between May 2009 and May 2010, we had quite a strong El Niño. When the mini-global warming from El Niño is added on, we really start to break records when the heat starts coming out of the ocean, as in July of last year. There was record-breaking flooding in China and India and then in August in Pakistan. Last September was the wettest on record in Australia. Then there was record-breaking flooding in Queensland, Australia, from December 2010 into January 2011. Those events were associated with quite high sea surface temperatures in the Indian Ocean and were partly a consequence of the El Niño and the change in ocean circulation. The residual effects of the El Niño meant there was extra evaporation and extra moisture available for the monsoons — first in Asia and subsequently in Australia.
M-M: Have these El Niños become more pronounced due to global warming?
KT: We have records of El Niño going back some 150 years, and there was a change in how it evolved starting somewhere around 1976. Today, the impacts of El Niño due to the floods and droughts around the world are certainly bigger. So while there are floods in South Asia and Australia, there are droughts in Africa and Peru and Equador. During El Niño, it tends to be wetter across the southern U.S. and drier in the north.
M-M: So, climate change really manifests itself with more weather extremes?
KT: Because water vapor in the atmosphere acts as a fuel for weather systems, the weather systems can get stronger. We can get heavier rains out of it. The two-day rains last May in Nashville, Tenn., were astounding. As for snow, if it’s a bit warmer but still below freezing, you can actually get more snow, as with some of the East Coast snow storms. Because there’s more moisture in the atmosphere, it gets dumped down in the form of snow instead of rain.
M-M: And you’re saying there’s more moisture in the air because globally the air is warmer?
KT: That’s correct — in particular over the oceans. The main hydrological source of moisture is the oceans. The fact that sea temperatures are half to one degree centigrade above what they were 30 years ago is enough to bolster the snow amounts by at least 10 percent, if not more.
M-M: Are you at all wary of interpreting extreme weather events as evidence for longer-term climate change?
KT: Yes, but it is the main way climate change is expressed.
M-M: How effective is comparative paleo-climatology, in other words, the study of ice cores and historical records, in helping your group understand the current climate?
KT: It helps, but the paleo data have large uncertainties and require careful interpretation. There’s an infinite variety in weather, and some of it occurs on relatively long timescales. The thing that is most clear is the increasing temperatures. Along with that, there’s more water vapor in the atmosphere. Beyond that, we can look to sea levels rising, Arctic sea ice and glaciers melting. All this is consistent with this warming. There’s also pretty good evidence that when it rains, it rains harder than it used to especially across the U.S. That’s due to anthropogenic climate change, and that’s only going to get bigger.
M-M: Is the climate definitely changing?
KT: The climate is continuing to change. One way change is manifested is through these extreme events. These are the ones that impact ecosystems and society. But because they are extreme, they are relatively rare and statistics on them are not good, but they are consistent with climate change. Because of that warming, there’s more water vapor in the atmosphere.
M-M: What’s needed in terms of resources or technology to improve long-term climate models?
KT: Warming has been well verified, but the models have major limitations, and regional climate is not very predictable. So, we need better observations and treatment of them. This includes assimilation of the data and its analysis into global gridded fields. The models also need to be improved.
M-M: What can we expect in the next couple of years?
KT: Because these extreme events lie outside what has been planned for, things like water systems and dikes and dams can be broken, as [seen] with Hurricane Katrina. The rains now are heavier than they used to be, so levees along the Mississippi and in California have also been breached. I expect that we will continue to see things we’ve never seen before.